Liquid crystal displays are commonly used as display devices for compact electronic apparatuses, not only because they provide good quality images but also because they are very thin. Liquid crystal in a liquid crystal display does not emit any light itself. The liquid crystal requires a light source so as to be able to clearly and sharply display text and images. Therefore, a typical liquid crystal display needs an accompanying backlight module.
Referring to FIG. 7, a typical backlight module 1 includes a light source 10 and a rectangular light guide plate 11. The light guide plate 11 includes a side light incident surface 111, a bottom surface 112 perpendicularly connected with the light incident surface 111, a top light emitting surface 113, and a side surface 114 far from and parallel to the light incident surface 111. That is, the light incident surface 111 and the side surface 114 are between the bottom surface 112 and the light emitting surface 113. The light incident surface 111, the bottom surface 112, the light emitting surface 113, and the side surface 114 are planar and smooth. The light source 10 is located adjacent to the light incident surface 111 of the light guide plate 11.
Light beams from the light source 10 enter the light guide plate 11 through the light incident surface 111. Because the bottom surface 112 is planar and smooth, most of the light beams from the light source 10 reach the bottom surface 112, and are reflected toward the side surface 114 and the light emitting surface 113. The light beams reaching the side surface 114 are further reflected toward the light emitting surface 113. Some of the light beams reaching the light emitting surface 113 are refracted by the light emitting surface 113, because the angles of incidence of these light beams are less than a critical angle of light beams at the light emitting surface 113. The refracted light beams directly emit from the light guide plate 11 through the light emitting surface 113. Other light beams reaching the light emitting surface 113 are totally reflected by the light emitting surface 113, because the angles of incidence of these light beams are greater than the critical angle. The reflected light beams finally emit from the light guide plate 11 through the light emitting surface 113 after being further reflected two or more times within the light guide plate 11. That is, many light beams have long propagation distances within the light guide plate 11 before they finally emit from the light guide plate 11.
The longer the propagation distances of the light beams, the lower the ratio of light utilization of the backlight module 1. In some cases, the backlight module 1 may be considered to have an unsatisfactory ratio of light utilization. Therefore more light sources are needed, or a high-powered light source is needed, to achieve a desired brightness of light beams output from the backlight module 1. However, the additional light sources or the high-powered light source typically result in other problems such as more power consumption, more heat generated, and a higher cost of manufacturing the backlight module 1.
What is needed, therefore, is a backlight module that can circumvent, overcome or at least mitigate the above-described difficulties. What is also needed is a liquid crystal display including the backlight module.